On Tue, Oct 10, 2006 at 11:16:27AM +1000, Benjamin Herrenschmidt wrote:> And the last of my "issues" here:> > get_user_pages() can't handle pfn mappings, thus access_process_vm()> can't, and thus ptrace can't. When they were limited to dodgy /dev/mem> things, it was probably ok. But with more drivers needing that, like the> DRM, sound drivers, and now with SPU problem state registers and local> store mapped that way, it's becoming a real issues to be unable to> access any of those mappings from gdb.> > The "easy" way out I can see, but it may have all sort of bad side> effects I haven't thought about at this point, is to switch the mm in> access_process_vm (at least if it's hitting such a VMA).

Switch the mm and do a copy_from_user? (rather than the GUP).Sounds pretty ugly :P

Can you do a get_user_pfns, and do a copy_from_user on the pfnaddresses? In other words, is the memory / mmio at the end of agiven address the same from the perspective of any process? Itis for physical memory of course, which is why get_user_pagesworks...

> That means that the ptracing process will temporarily be running in the> kernel using a task->active_mm different from task->mm which might have> funny side effects due to assumptions that this won't happen here or> there, though I don't see any fundamental reasons why it couldn't be> made to work.> > That do you guys think ? Any better idea ? The problem with mappings> like what SPUfs or the DRM want is that they can change (be remapped> between HW and backup memory, as described in previous emails), thus we> don't want to get struct pages even if available and peek at them as> they might not be valid anymore, same with PFNs (we could imagine> ioremap'ing those PFN's but that would be racy too). The only way that> is guaranteed not to be racy is to do exactly what a user do, that is do> user accesses via the target process vm itself....

What if you hold your per-object lock over the operation? (I guessit would have to nest *inside* mmap_sem, but that should be OK).-To unsubscribe from this list: send the line "unsubscribe linux-kernel" inthe body of a message to majordomo@vger.kernel.orgMore majordomo info at http://vger.kernel.org/majordomo-info.htmlPlease read the FAQ at http://www.tux.org/lkml/